Visual fields interpretation in glaucoma: a focus on static automated perimetry.

Despite all the treatments, operations, and medication at our disposal, there is still a significant number of people whose sight we cannot fully restore. What happens to these patients once they leave our care? Without the necessary support, advice, and low vision devices, their remaining vision will not be very good; this can make life a struggle. Support may be difficult to find, as low vision services are often inadequate or inaccessible in many lowand middleincome countries. Professionals, such as rehabilitation workers, ophthalmologists, mid-level eye care workers, optometrists/ refractionists, and special education teachers, may not know what to do about people with low vision, leaving them with no-one else to turn to. Individuals who can only see light or movement of large objects will need rehabilitation that focuses on non-visual strategies for learning and daily tasks. However, there are many people who have slightly better vision, but are still classified as blind, who have the potential to use their sight. These people could benefit from low vision care, which may include refraction, provision of magnifiers, and/or environmental modifications. The World Health Organization defines a person who needs to be assessed for low vision care as someone “who has impairment of visual functioning even after treatment and/or standard refractive correction, and has a visual acuity of less than 6/18 down to and including light perception, or a visual field of less than 10 degrees from the point of fixation, but who uses, or is potentially able to use, vision for the planning and/or execution of task.” The important part of this definition is that people should only be assessed for low vision interventions once all other treatments the person needs (surgical, medical and/or optical) have been given. The definition also emphasises the importance of vision for day-to-day functioning. People who may be able to benefit from low vision care will want to do a range of different things. In many lowand middleincome countries, for example, many people with low vision are aged over 50 years and cannot read or write. They will have different needs, and require different services, compared to children or adults in employment. Low vision has a significant impact on people’s lives. People with low vision may struggle to look after themselves without help. Having low vision affects their status in the eyes of others and can make social situations difficult. It reduces the ability of people to pursue an education, to look after their children, and to earn an income. People with low vision are also at greater risk of falls and death. With our support, people with low vision can make better use of their sight to do the things they want and need to do. We hope this issue will show you how.

1 Manual and/or automated VF plotting is a subjective test. So, if you get meaningless results, it could be due to improper subject selection, or poor patient understanding of how to perform the test. 2 The VF reflects changes in the visual pathway.
It does not diagnose glaucoma. One should always look carefully at the fundus, to see if the VF defects match the appearance of the disc and retina or not. The presence of neuro-retinal rim pallor, for instance, excludes glaucoma as the only pathology responsible for the fi eld defects (even in the presence of a confi rmed glaucoma diagnosis). 3 While visual acuity is the single most important disease progression parameter for patients, and visual fi elds are still the most important disease progression parameter for physicians, the two tests cannot be separated when one looks at the visual function and the functional reserve of the eye. After all, our patients are concerned with what they see, without separating their visual acuity from their visual fi elds. Like most glaucoma specialists, I have had cases of advanced glaucoma where only a temporal crescent remains. Those patients have developed ways of using their existing fi eld, and have carried on using it to perform life activities. It is our duty to help them keep their functional vision, despite the extensive loss. Figure 1. The three-dimensional conical structure of the visual fi eld. For the right eye, the boundaries of the fi eld are 100 degrees temporal and 60 degrees upper, lower, and nasal. The apex of the cone is at the nodal point of the eye, where light rays converge on their way to the retina, forming the perimetric angle.

VISUAL FIELDS INTERPRETATION IN GLAUCOMA
Normally, these two terms may be used synonymously. However, in some pathological conditions, certain areas of the fi eld of view may not be perceived (seen), and a fi eld defect results. These defects are either "relative" or "absolute", depending on the degree of damage caused by the pathology, and the remaining functional reserve of the visual system.
In primary open angle glaucoma (POAG), the development of these defects is usually slow, and may be masked by the overlapped visual fi elds of both eyes to produce a single binocular fi eld. Indeed, up to 40% of the retinal ganglion cells may be lost, before any VF changes could be detected.
The presence of more than one pathology aff ecting the visual pathway (with superimposed multiple defects) may complicate our interpretation of fi eld testing. This is important, since most patients diagnosed with POAG present at an age where by other factors may also have an eff ect on the visual apparatus.

Visual fi eld boundaries
The VF is a three dimensional cone (Traquair's Island of vision), with its apex at the nodal point of the eye, and its base at infi nity (or at whatever distance we plot it by a perimeter screen or bowl) ( Figure 1). The purpose of visual fi eld testing is to defi ne the topography of the island of vision to recognize any variation from normal.
The plotted VF (the base of the cone) extends for approximately 60 degrees superior, inferior, and nasal and 100 degrees temporally. For practical purposes, the VF plot may be divided into three major parts: the central 30 degrees, the peripheral fi eld (from 30 to 60 degrees), and the temporal crescent ( Figure 2).
In the large part of our practice today, automated static perimeters are used to test the central VF. The central VF reflects the function of approximately 66% of the retinal ganglion cells, and its cortical representation occupies 83% of the visual cortex. Thus, exploring it could reveal almost any pathology in the visual pathway ( Figure 3).
These days, the need to explore the peripheral visual fi eld is limited to searching for a ring scotoma, confi rming a nasal step defect, and maybe looking at the temporal crescent. These defects can easily be discovered using a tangent screen and/or careful confrontation techniques. Investing in an ultramodern automated kinetic perimeter may thus be avoided, especially in the face of limited resources.

Visual fi eld centre
The horizontal axis of the visual fi eld is mostly represented by the horizontal raphé of the retina. The vertical axis, however, is represented by two boundaries, a short vertical line through the foveal area (fi bres nasal to it go to the nasal half of the disc), and a semi-vertical line through the centre of the disc, separating the remaining nasal fi bres from the temporal fi bres of the retina (Figure 4).
This fact should always be remembered, since it explains the reason for "Band Atrophy" of the optic disc in lateral geniculate body lesions, which also produce VF defects (sectoranopias) mimicking glaucomatous defects.

Examination strategies
Static perimeters test the diff erential light sensitivities of specifi c retinal locations distributed on a fi xed grid pattern. The spacing between these locations (points) varies according to the examination area targeted. When testing the central 30 degrees and central 24 degrees with the Humphrey perimeter, the grid points are spaced 6 degrees apart. The central horizontal points may be distributed on the horizontal axis (programme 30-1 or 24-1), or at equal distances from the horizontal axis (programme 30-2 or 24-2). The advantage of using the central 30-2 or 24-2 programmes lies in the spacing of the central horizontal points at 3 degrees from the centre of the fi eld, with greater sensitivity to changes across the horizontal retinal raphé (nasal steps) ( Figure 5).   At the fovea, a vertical line separates the narrow band of nasal fi bres (which join the nasal half of the optic disc), from the temporal arcuate fi bres which join the superior and inferior poles of the optic disc. The horizontal raphe is at the horizontal axis.The light grey areas on the disc show the development of Bow-Tie (Band) atrophy in Chiasmal lesions. In this case the pallor will involve both the neuroretinal rim and the cup at the 9 and 3 o'clock positions.

VISUAL FIELDS INTERPRETATION IN GLAUCOMA
One of the most useful programmes, particularly in advanced glaucoma, is the central 10-2 programme. The test locations are 2 degrees apart, and a large number of points are crowded in the central 10 degrees. This gives a magnifi cation eff ect, and shows the relationship between the central fi eld defects and the point of fi xation more precisely. This may reflect on the decision to operate or not, and helps with estimation of the risk of postoperative wipe-out ( Figure 6).

The Humphrey printout
The test results (with or without the statistical analysis) are usually printed on one sheet called the printout (Figure 7). When examining the test results, you MUST hold the two printouts (right eye print out in your right hand facing your right eye, and left eye printout in your left hand facing your left eye), and examine them together ( Figure 8).

Patient's data and test parameters
These are at the top of the printout.
• Your fi rst priority is to make sure that those fi elds belong to the patient in question, and that the specifi c program requested has been used, with the parameters that you have asked for. • The date of birth has to be correct (for comparison with range of normal values for age). A pupil diameter of at least 2.5 mm is essential to avoid overall depression of test values. Finally, using near correction lenses, and/or high astigmatic error correction lenses is strongly advisable to help the patient appreciation of the test targets. A frameless lens may be best suited for that, but other lenses could be used and allowance for any rim artefacts made during interpretation.

Reliability indices
The second priority is to check your patient' performance (reliability). These parameters are printed at the top left hand side of the prin out In the Humphrey perimeter, and at the bottom right hand side for the Octopus perimeter.
• Fixation loss. Normally this is between 0 to 2%. If the loss exceeds 20 %, this is generally considered as poor reliability. However, it may also be an indicator of advanced glaucoma with an abnormally large blind spot. Other indicators of advanced glaucoma should be looked up fi rst, before disqualifying the test results as unreliable based on poor fi xation. • False-positive or false-negative responses.
Scores in excess of 20-30% indicate a test of questionable reliability.

VISUAL FIELDS INTERPRETATION IN GLAUCOMA
The grey scale • This is a graphic representation of the recorded threshold sensitivities in the numeric scale.
Regions of decreased sensitivities are displayed in darker tones. The grey scale plot is very useful for displaying patterns of loss (nerve fi bre layer defects versus neurological defects) and thus should be inspected fi rst.
When looking for such patterns, both right and left grey scales should be inspected together.
If any visual fi eld defect related to pathology of the visual pathway distal to the lamina cribrosa is revealed, then attention is directed to other tests to examine the rest of the visual pathway. • It is also a very useful tool in explaining -to the patient and family -the stage of the disease and its progress over time.

The numeric scale
This is the main test result. It shows the retinal sensitivities at the diff erent test locations, expressed in Decibels (dB). In the Humphrey perimeter, 10 points are retested, and in those points, the two results are printed next to each other. This is done to determine the short term fluctuation (see later). In the Octopus perimeter, retesting is done in all test locations. The numbers expressed in the numeric scale may be looked at as heights, and with the higher values in the centre, and the least values at the periphery, one can 'see' the centre of the hill of vision in a three dimensional way.

Statistical analysis of test results
Inclusion of statistical analysis software is the reason for the widespread popularity of static perimetry. Computers can store a huge amount of numbers and use them to look for patterns, compare them with stored data bases, and perform all kinds of analysis on them.
The Statpac software is a commonly used analysis tool in both Humphrey and Octopus machines. It is used to look at suspicious clusters of numbers, analyse them, and monitor their change over time.

Total deviation plot
• These numbers show the diff erence (in dB) between the test results and the normal values expected for the patient's age group. 0 dB = no diff erence (normal), while -13 dB = large depression from normal value (see the total deviation plot in Figure 7). The true value of the defect in this instance should be -17 dB, but the computer allows for a variability of -4 dB. • This is in fact a plot of the probability of each point change being normal. If less than 0.5% (indicated by the solid black squares), then the point change is highly unlikely to be normal.

Pattern deviation plot
This particular analysis tool is helpful for detecting visual fi eld defects (scotomata) in the presence of media opacities, such as cataract. It does this by looking at the overall sensitivity changes in the hill of vision (the 'pattern' here is the conical shape of the hill of vision). If there is an overall depression (all test values are reduced from normal due to cataract), then it will subtract this value from all test points, leaving behind clustered fi eld loss (localised defects), which may be due to glaucoma.

Global indices
These numbers represent mathematical summaries of all the sensitivity values produced by the test. They are useful tools for having a quick idea about the entire fi eld, and sequentially compare test results for the same eye (change analysis). Looking at them however, does not replace examining other test Continues overleaf Figure 8. Examining the central VF grey scale of the right eye alone may be confusing, as there is no specifi c pattern to the fi eld defect. However, looking at the two grey scales together reveals a left central fi eld defect, signifying optic neuritis. In this case, looking at the rest of the printout data (statistical package) may be useless, as it is designed to analyse glaucoma defects only.
'Inclusion of statistical analysis software is the reason for the widespread popularity of static perimetry'

Mean deviation
This number reflects the overall depression (deviation from normal values) of the fi eld. All the obtained values of the test are added, and divided on the number of test locations. This gives the mean value of the test. The same is done for the normal expected values stored in the computer data base. The diff erence between the 2 values represents the MD. Normally it should not exceed -2 dB. If the MD is significantly outside the normal, then a P value is assigned to it.

Pattern standard deviation
For practical purposes, the pattern standard deviation (PSD) reflects the degree of departure (diff erence) of the measured VF pattern (shape) from the normal hill of vision. A small PSD reflects a smooth uniform hill of vision, while a large PSD value reflects an irregular hill of vision (Figure 9).

Short-term fluctuation
The short-term fluctuation (SF) value reflects diff erence in the response for certain test spots on re-testing. It is used to refi ne the test results by excluding errors due to patient fatigue. Normally it should not exceed 2 dB. However, high SF may be a sign of early glaucoma.

Corrected pattern standard deviation
An irregular hill of vision revealed by a high PSD value may be due to low patient reliability, actual VF defect, or both. By removing patient fatigability factors (correcting the PSD by subtracting SF value), the true shape of the hill of vision is appreciated; this is the corrected pattern standard deviation (CPSD) (Figure 10).

The change analysis printout
This is a simple graphic representation of summaries of sequential VF tests for the same eye using all global indices. It allows you to track changes over time, and evaluate the progression (or stability) of the glaucomatous process. Care should be taken when using change analysis, because it requires many reliable fi elds (at least 4 excluding the fi rst one to avoid learning mistakes). To compare results from a small number of fi elds, one can use the 'Multiple fi elds printout' , to visually track changes between the tests in question.

The glaucoma hemifi eld analysis
This software allows comparison of VF defects across the horizontal axis (looking for -and comparing -nasal steps). As such it alerts you to the need to re-examine the printed results, looking for such diff erences. The three important responses to look for are: 1 Within normal limits (no diff erences) 2 Borderline (early diff erences) 3 Outside normal limits (obvious diff erences between the upper and lower halves of the fi eld) In most instances, such diff erences are seen in the other graphical plots.

Notes on the Octopus printout
Data representations in the Octopus printout are -for the most part -similar to those in the Humphrey printout. Patient and examination data are at the top of the printout, followed by the value table (numeric scale) and the grey scale (which Figure 9a. The pattern standard deviation (PSD) is the diff erence between the expected data (the straight line at the top) and the patient's own data (the irregular line beneath it). Here, the sum of the diff erences is 38, which, when divided by number of test locations, yields 5.4. This reflects an irregular hill of vision. Figure 9b. The sum of the diff erences is here is 1.7, a much smaller number, which reflects a much smoother hill of vision with a more normal pattern to the visual fi eld.
'The change analysis printout allows you to track changes over time' PSD = 12/7 = 1.7 PSD = 37/7 = 5.4 is coloured). These are followed by a comparison table and a corrected comparison table (total deviation and pattern deviation). The probability plots for those tables follow. Finally the visual fi eld indices (global indices) are at the bottom right hand side of the printout page.
The indices are MS (mean sensitivity) which is the mean value of all test results, MD (mean defect) which is the same as mean deviation, LV (lost variance) which is the same as pattern standard deviation, and CLV (corrected lost variance, which is the same as corrected pattern standard deviation). The RF (reliability factor) is a score of patient's reliability (0 to 15%).
The major diff erences between the two printouts are the presence of the Bebie curve, and phasing.
The Bebie curve is a cumulative defect curve that allows ranking of VF defects (it looks only at the defects, not the fi eld). In the presence of established glaucoma diagnosis, this is useful to monitor the progression of both overall depression and localised defects (Figure 11).
Staging VF testing is a useful feature. It allows collecting examination results from points considered most important for glaucoma diagnosis in the fi rst and second stages (80% of the test). This allows printing of these results and stopping the testing process, in case of patient fatigue. In all other perimeters, the entire testing process has to be completed before printing of the results.

Monitoring visual fi elds in advanced glaucoma
Two major problems arise in patients with advanced fi eld loss in glaucoma: 1 Threat to fi xation. The central progression of VF defects towards fi xation is an ominous Figure 10a. Removing the eff ect of the short term fluctuations (SF), reveals the true diff erence (in green) between the regular "expected" pattern of the hill of vision, and the patient's hill of vision.  . This cumulative defect curve shows both generalised depression of the fi eld values, as well as a large deep localised defect (seen in the grey scale). It doesn't however signify the type of the defect.

Continues overleaf
Tilted discs. These usually give superior arcuate defects similar to those produced in glaucoma. However, they are peripheral, and not related to the blind spot. If on the other hand glaucoma developed in an eye with a tilted disc, the resulting defects relate to the blind spot, and are central. Careful inspection of the optic disc will allow for correct interpretation of the fi eld results.
Central scotoma due to optic neuritis. This is a papillomacular bundle defect that crosses the horizontal line. It is associated with reduced visual acuity, and reduced colour vision. It regresses with treatment, but remains central.
Bitemporal hemianopic defect due to chiasmal lesions. These can easily be confused with glaucoma, especially if the two pathologies coexist in the same patient (glaucoma plus pituitary tumour) ( Figure 13). Figure 13a. These are the right and left visual fi elds of a 61 year-old patient, who was diagnosed with POAG 15 years prior to presentation. Both optic discs were terminally cupped. The IOP was poorly controlled in both eyes; however, the patient was aware that his right eye was worse than the left eye.

Diff erential diagnosis of glaucomatous visual fi eld defects
Looking at the VF of the left eye, there is a clear temporal fi eld defect respecting the vertical midline. sign of progression. Most surgeons fear this because it may be related to a higher risk of visual loss following surgery for glaucoma. To allow for better interpretation of these fi elds, one must use the Central 10-2 program available in almost all threshold testing perimeters. This allows "Zooming in" and magnifi cation of the central fi eld, revealing the true relationship between fi eld defects and fi xation point. This may allow for better planning for future surgeries in advanced glaucoma ( Figure 6). 2 Depression. As a progressive blinding disease, the psychological impact of glaucoma on patients cannot be overemphasised. This aff ects their performance in VF testing, as well as their compliance with medications, and their quality of life. To help patients perform better in VF testing, and improve their moral, one may use a larger target size (s ize IV) for testing.
The standard size III in SAP (standard automated perimetry) is mainly used for easier and faster mapping of the blind spot. Its use in all VF testing is not a law! Using size IV is compatible with all analysis software. Moreover, those patients are well trained in VF testing, and looking at their new test results may improve their depression and encourage their compliance with medications ( Figure 12).